1. Field of the Invention
The present invention relates to capacitor equipment which comprises a series-connection, connected to an alternating voltage network, of
A. two anti-parallel-connected thyristors, PA1 B. one capacitor PA1 C. one reactor, a control device being connected to the control electrodes of the thyristors and arranged to ignite the thristors alternately at times in the vicinity of the peak value of the alternating voltage.
2. The Prior Art
Capacitor equipment of this kind is known and it is used, for example, for power-factor correcting purposes, that is, for compensation of a varying reactive current which is consumed by a load object. Usually a plurality of series-connections are then connected in parallel with each other to the network, and the number of capacitors which are connected at each moment is determined by the need for compensation at the time in question. Suitably a reactive power-sensing member can be arranged to sense the reactive power consumed by the load object and, in dependence thereon, to control the connection and disconnection of the capacitors.
Connection of a capacitor is performed by igniting the thyristors connected in series with the capacitor, and suitably at such a time that the voltage to which the capacitor is charged is as great as the instantaneous value of the alternating voltage.
Disconnection of a capacitor is achieved by removing the ignition signal to the thyristors, so that the thyristor which is conducting at the time is extinguished at the next zero passage of the capacitor current.
Each capacitor may, of course, consist of a battery of series and/or parallel-connected capacitors, which is most common in practice.
The whole capacitor equipment may be connected to the network by way of a transformer in order to obtain a suitable level of the current and the voltage for the components included in the equipment. Suitably a circuit breaker is arranged between the equipment and the network to disconnect the equipment in case of a fault.
The reactor mentioned in the introduction is usually arranged in series with each capacitor, among other things to limit the current derivative when connecting the capacitor, and thus to protect the thyristors.
In a three-phase network, capacitor equipment of the type described may be constructed with three phases, in which case capacitors are connected between each pair of conductors. It will thus also be possible to compensate for an unsymmetrical reactive power consumption.
In thyristor equipment of this kind, it is inevitable that a so-called misfiring will occur at some times, which cannot be completely prevented. A misfiring occurs when a thryistor is ignited at a time when it should not be ignited. Misfirings may be caused by transient voltages and other disturbances, or by malfunctioning of the control devices of the thyristor.
In capacitor equipment of the type described, a misfiring may have serious consequences. As will be described below, it gives rise to overcurrents and overvoltages, which may be of such a magnitude that even amply dimensioned components are destroyed. This applies primarily to the thyristors included in the equipment.